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This is an artist's impression of Herschel’s superfluid helium tank.
To protect the sensitive instruments from heat generated during operations and to achieve its challenging objectives, the satellite must operate at very low temperatures. This is why the spacecraft’s brain – or its payload module – hosts a cryostat, a cryogenic module inside which the cold components of the scientific instruments are mounted.
Inside the cryostat, the sensitive instrument detectors are cooled down to about -273ºC (0.3 degrees above absolute zero). This low temperature is achieved using superfluid helium (at about -271ºC) and an additional cooling stage inside the focal plane units.
Inside the tank, helium is kept in its superfluid state at its boiling point (1.65 K or –271.5°C). The helium liquid and gas cools the focal plane units of the instruments and the shields. The liquid boils and a porous plug allows the separation of the liquid from the gas such that only gas leaves the tank. It slowly flows from the tank into pipes around the payload to cool it to between 1.7 K (–271.4°C), 4 K (–269°C) and about 10 K (-263°C).
The gas then continues into the rings of three thermal shields to cool them to 30K (–241°C), 50 K (–221°C) and 60 K (–211°C), respectively.
The gas is then finally released in space. The cryostat vacuum vessel is facing cold space and radiatively cools to about 70 K (–203°C).